The prior art of leak detection is adequately described in the 1976 publication Handbook of Vacuum Leak Detection published by the American Vacuum Society. Conventionally when leak detecting a vessel it is most common to connect the input port of the leak detector to the vessel under test and evacuate it to a low pressure using a mechanical pump. Further pressure reduction is achieved by use of a diffusion pump in combination with a liquid nitrogen cooled cold trap which freezes out water vapor, carbon dioxide and hydrocarbons including oil from the diffusion pump so they do not contaminate the mass spectrometer leak detector device. The mass spectrometer leak detector has means for ionizing the detecting gas, which is usually helium, accelerating it in a well defined beam, bending the beam by means of a magnet and/or a direct current and/or an alternating current electrostatic field which spreads out the beam proportional to molecular mass, and detecting the ion beam for the tracer gas.
To find a leak in the vessel which is being tested, one sprays a small stream of tracer gas over the outside of the vessel. Tracer gas drawn into the vessel diffuses rapidly to the leak detector where it causes an increase in the output signal. The partial pressure of the tracer gas at the leak detector is a balance between the rate at which the tracer gas is leaking into the system and the rate at which the diffusion pump removes it.
Another method of leak detecting is to fill the vessel with tracer gas and probe the outside with a sniffer connected to the input of the leak detector. The leak detector vacuum pump system is used to draw a small stream of air from the sniffer probe to the mass spectrometer leak detector device. Tracer gas scaping through a leak in the vessel is thus detected by the leak detector. This technique requires a low background of tracer gas in the environment in order for the tracer gas from the leak to be detected.
Conventional mass spectrometer leak detectors employ a diffusion pump and a liquid nitrogen cold trap to remove certain gases from the testing atmosphere so that the instrument can sense the presence of helium at very low levels. In other prior art devices cryopumps have been used to create ultra-low vacuums. The art of cryopumping (cryogenic pumping) is adequately set out in the specification of U.S. Pat. No. 4,150,549, the specification of which is incorporated herein by reference. In the '549 patent, it is pointed out that cryopumping devices have used three surfaces to remove different gases from the vacuum environment. These surfaces generally have been cooled to various temperatures below 120.degree. K. (Kelvin). These surfaces have been used to remove water and carbon dioxide (by freezing in a temperature range of 40.degree.-120.degree. K.) nitrogen, oxygen, argon, carbon monoxide, methane, and halogenated hydrocarbons (by freezing at temperatures between 10.degree. K. and 25.degree. K.) and helium, hydrogen, and neon (by cryosorption at temperatures of 10.degree. to 2.degree. K.). Cryosorption is adsorbing gases in a sorbent at cryogenic temperatures as shown in the apparatus of the '549 patent.